Background: We assessed diurnal variation in the direction of mood switches in a sample of outpatients with rapid-cycling bipolar disorder who were on stable medication regimens. We predicted that patients would be more likely to switch from depression into mania or hypomania during the daytime hours and from mania/hypomania into depression overnight.

Method: Fifteen patients with rapid-cycling bipolar disorder completed self-rated mood scales twice a day: once shortly after awakening and once at bedtime. Using 3 months of data for each patient, we performed categorical analyses (McNemar chi-square) to study the direction of mood switches between each day’s morning and evening rating and between each evening rating and the subsequent morning rating.

Results: As predicted, switches that occurred between the morning and evening ratings were more likely to be from depression into mania/ hypomania or euthymia (64.3%) than in the opposite direction (35.6%; p < .0001). Similarly, switches that occurred between the evening rating and the next morning’s ratings were more likely to be from mania/hypomania or euthymia into depression (64.8%) than in the opposite direction (35.2%; p < .0001).

Conclusion: Extended wakefulness, exposure to light, increased activity, and/or endogenous rhythms could contribute to the elevation of mood during the course of the day. Sleep, darkness, reduced activity, and/or endogenous rhythms could contribute to the tendency to switch into depression overnight. Clinicians should attend to the time of day that clinical assessments are performed in patients with rapid-cycling bipolar disorder. Potential therapeutic implications include the use of light or activity during depression and use of induced sleep or exposure to darkness during mania/hypomania.